1. Field of the Invention
The present invention relates to a constant-voltage generation circuit which is manufactured in a standard CMOS process technology; a standard output voltage of which is determined by a bandgap voltage of transistors, and more particularly to a constant-voltage generation circuit employing operational amplifiers operated by a single power source.
2. Description of the Related Art
A constant-voltage generation circuit preserves its standard output voltage at a constant level, though the inputted voltage varies due to temperature variation.
A conventional constant-voltage generation circuit is well known, for example, disclosed in Japanese Patent Laid-Open Publication NO. 61-217815.
The conventional circuit includes a reference voltage generation circuit and amplifiers, in which the reference voltage generation circuit is connected to power source voltage V.sub.DD and outputs reference voltages V1 and V2 by inputting standard output voltage VR of the output of the circuit.
The operational amplifiers output standard output voltage VR by differentially amplifying reference voltages V1 and V2.
The reference voltage generation circuit, which incorporates two transistors operationally controlled by standard output voltage VR and a plurality of resistors, outputs reference voltages V1 and V2 based upon the bandgap voltage of these transistors.
Two transistors and the plurality of resistors of the reference voltage generation circuit are connected to each other in such a manner that a reference voltage satisfying V1&lt;Vs or V1&gt;V2 is outputted, respectively, depending on standard voltage VR being either higher or lower.
The operational amplifiers are so constructed that the non-inverted input terminal is connected to reference voltage V1 and the inverted input terminal is connected to reference voltage V2, respectively.
The operational amplifier outputs at its output terminal standard voltage VR which is amplified in proportion to the voltage between the two input terminals.
It is well known as such the operational amplifiers include constituted either a combination of a PNP-MOS transistor and an NPN-MOS transistor or an NPN-MOS transistor and a PNP-MOS transistor.
In both cases, the operational amplifiers are connected between power source voltage V.sub.DD and negative power source voltage V.sub.SS, in which the prescribed transistors differentially amplify reference voltages V1 and V2.
In the above described constant-voltage generation circuit, when standard output voltage VR increases as a result of a change temperature, reference voltage V1 becomes lower than reference voltage V2, whereby the supply voltage to the non-inverted terminal of the operational amplifier becomes lower than the voltage supplied to the inverted input terminal so that standard output voltage VR drops.
In this manner, the constant-voltage generation circuit compensates standard output voltage VR so as to output stabilized constant standard output voltage VR.
The constant-voltage generation circuit according to the conventional technology has the following problems set forth below to be improved.
The requirement for low power consumption restricts two transistors in the reference voltage generation circuit to be supplied with high current.
Also, the requirement for minimizing package area restricted the resistor employed in the reference voltage generation circuit to a large resistance value.
As the result, the reference voltage is set to nearly ground voltage GND.
Accordingly, since reference voltages V1 and V2 become less than a threshold voltage of the differential amplification transistor of the operational amplifier, the transistor turns off occasionally.
In order to preserve the threshold voltage of the transistor, negative power supply voltage V.sub.SS of the operational amplifier must be set lower than ground voltage GND.
For the reasons set forth above, the conventional constant-voltage generation circuit requires another negative power supply voltage V.sub.SS in addition to normal power supply voltage V.sub.DD and ground voltage GND.